This invention relates to alternating current, superconductive electrical machines and, more particularly, to the field windings used in both generators and motors of this type.
The windings used in AC electrical machinery can be classified in two groups: armature windings and field windings. An armature winding is the main current-carrying winding in which the electromotive force is induced. The current in the armature winding is known as the armature current. A field winding is the winding that produces the magnetic field in the machine. The current in the field winding is known as the field or exciting current.
The armature winding in AC synchronous motors and generators is normally constructed on a stator, and the field winding is in the rotor. However, in special cases, the armature winding could be located on the rotor and the field winding can be on the stator.
To describe the environment of the present invention, one example of a superconductive AC machine is illustrated in FIG. 1. This machine is an alternating current, synchronous generator having two poles and a superconductive main field winding. In one design the machine has a capacity of 1200-MVA with an output of 23.5-KV at 3600-RPM. This machine is adapted for connection to a public utility power system.
The AC generator of FIG. 1 includes a rotor 6 that is supported by two bearings 10 in a housing 7. The rotor turns within the stator bars 8 that constitute the armature winding. The rotor is turned by a turbine (not shown) attached to the coupling 12. The field winding in the rotor is energized through the exciter 14.
The rotor 6 contains a superconductive field winding 16 which is cooled to the liquid helium operating temperature of 4.2.degree. K. The winding is housed within an electromagnetic shield 18 that also serves as a vacuum envelope for the winding. The interior of the rotor at points 19 is permanently evacuated in order to insulate the rotor from other components of the generator operating at ambient temperatures. The electromagnetic shield screens the superconducting winding from non-synchronous components of the magnetic field produced by unbalanced or transient currents in the stator 8. Inside of the electromagnetic shield is a thermal radiation shield 20 which intercepts thermal radiation from the ambient temperature electromagnetic shield 18. The winding 16 is also protected at its ends by two thermal radiation shields 20, 21'.
Inside of the shields is a torque tube 23 which transmits torsional forces from the field winding 16 to the turbine coupling 12. The torque tube is illustrated in FIG. 2 in end elevation. Although different schemes for supporting the field winding are available, the apparatus shown in FIG. 2 has a field winding which consists of many winding modules. In particular, the torque tube houses nine superconductive winding modules 16. Each superconductive winding module is ractrack shaped, FIG. 1, and is manufactured from niobium-titanium filaments. Each winding module is supported in the torque tube by an outer housing 26, a series of support plates 28, and two aluminum pole segments 30, FIG. 2, located at opposite ends of the winding stack. The assembly of winding modules 16, housings 26, support plates 28, and pole segments 30 is fastened together by cross bolts 32 located along the longitudinal axis of the rotor. This assembly is inserted into the cylindrical torque tube 23 and is held in place by a plurality of keys (not shown). The torque tube is fabricated from a non-magnetic steel such as either an FeNiCr-base austenitic stainless steel or a nickel-base stainless steel.
Referring to FIG. 1, the field winding 16 is cooled by the flow of liquid helium through the rotor. Saturated liquid helium is delivered to a central supply tube 36 from a liquefier or supply dewar (not shown). The liquid helium flows along the axis of rotation of the rotor into the torque tube 23. The liquid helium is distributed in the rotor by a radial supply tube 38 and a level control monitor 39. When the rotor 6 turns, centrifugal force causes the liquid helium to assume the shape of a cylinder. The lighter weight helium vapor becomes centered about the axis of rotation of the rotor, FIG. 2, and the interface between the liquid and the vapor is indicated by reference numeral 46.
During operation of the generator the liquid helium boils as a result of the heat transferred into the cold region of the rotor. Two separate streams of boiled-off vapor are removed from the rotor. One stream passes through a series of spiral flow channels 41, then through passage 42 that runs across the inner side wall of the electromagnetic shield 18, and thereafter through the exhaust tube 43 which is concentric with the central supply tube 36. The other stream of vapor passes through a second plurality of spiral flow channels 41 which also connect to the concentric exhaust tube 43. The warm helium vapor thereafter flows out of the generator and is returned to the liquefier (not shown).
When the field winding 16 is cooled to a temperature of approximately 4.2.degree. K., the winding modules become superconductive. At this temperature the winding is non-resistive. However, the circuit that excites the field winding has some electrical resistance from the exterior leads to the generator and the field current power supply. Thus, the field winding circuit has a very large but finite time constant. As used herein the time constant of the field winding is the ratio of the self-inductance of the field winding to the resistance of the field winding circuit.
When the generator is connected to an electrical power distribution system such as a public utility, the generator must operate in synchronism with the other machines and loads in the system. The superconductive generator must accommodate for changes in the electrical load as rapidly as possible and in coordination with the other machines in the system. Generally, the active portion of the electrical load is accommodated by varying the mechanical power supplied to the generator from the turbine (not shown). The reactive power demand on the generator is satisfied by altering the excitation to the field winding.
Heretofore, one problem with superconductive machines and, in particular, superconductive AC generators, has been the inability to rapidly accommodate for changes in the load. The large time constant of the superconductive field winding prevents the field current from being instantaneously changed to match the electrical load placed on a generator. If the electrical load is not quickly matched by the output of the generator, the generator begins to oscillate and, if the condition is permitted to continue, the generator becomes dynamically unstable.
In the past large exciter voltages have been used on superconductive field windings in an attempt to change the field current rapidly. To obtain these large voltages, a large capacity exciter, one capable of handling very high voltages, has been used. These exciters are both costly to construct and difficult to maintain. In addition, the use of large exciter voltages has been constrained by the dielectric strength of the field winding insulation. Until now, the dielectric strength of the insulation has limited the magnitude of the voltage that can be impressed on the main field winding because the peak voltage is governed by the value of discharge resistor connected across the field winding terminals. Further, the dielectric strength of the insulation has also limited the rate at which changes in the excitation current can be made because the field winding terminal voltage is proportional to the time rate of change of the field winding current.